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Once you know what chipset(s) you can use, you need to make sure that the motherboard
will fit inside your case. Many manufacturers will offer essentially the same motherboard
in two different form factors to try to satisfy a
larger number of users. Other manufacturers may decide to offer only the form factor they
believe is the most popular in the market they are targeting with a particular model.
Because of this, the case you have decided upon will further reduce your choices. If you
have not yet decided upon a case, you may want to use some of the criteria outlined below
to choose your board so that the model you choose determines the form factor. Most of the
time, however, the motherboard will be fitting into an existing case or into a case that
has already been decided upon. If you are not familiar with form factors, the following
discussion may be of some help.

When IBM introduced the PC AT, they used a motherboard layout that became known as the AT form factor. Manufacturers of PC clones used a
smaller version of this layout, called the Baby AT, which is one of the two standard form
factors in use today. Variations of this are called the 2/3 Baby AT and ¾ Baby AT, which
differ from the "full" Baby AT (and each other) only in the length of the board.
All Baby AT boards are approximately 8.5" wide, with the lengths being about 13
inches (full), 10 inches (3/4) and 9 inches (2/3). After the Pentium processor was
introduced, Intel developed the ATX form factor
to "fix" several problems with the old AT boards. Essentially, they rotated the
board 90 degrees (approx. 12" wide and 8" or 9" deep), moved the CPU socket
and IDE connectors and added some nice power features (a 20-pin power connector vs. the
older pair of 6-pin connectors). Recently, the Micro ATX form factor has appeared, which
is still 12" wide, but is only about 5" to 7" deep. Micro ATX is used
primarily on motherboards with integrated sound, audio and/or SCSI since fewer expansion
slots are necessary. Though ATX was touted as being vastly superior, it has its detractors
and the majority of motherboards sold are still AT style. One of the reasons for this is
that the ATX form factor requires an ATX case, and many user upgrading from older machines
do not want to spend the extra money. Two other form factors that exist are LPX and NLX.
Both of these are used strictly by large OEMs and will not be available to the vast
majority of users.